1. Technical Field
This invention relates to the art of making and using catalysts for automotive use and, more particularly, to the art of making alumina-supported, precious metal catalysts, which support has been stabilized by the use of ceria.
2. Description of the Prior Art
The use of transition-type aluminas as a suitable support for automotive exhaust gas catalysts encounter at least one very significant problem. That is, as the temperature increases during use of such alumina supports, they undergo a phase change to alpha alumina. This instability is undesirable because it results in a severe loss of surface area and physical strength. These changes, in turn, lessen the effectiveness of the catalysts because of shrinkage, which causes lower activity and loss of catalyst due to attrition.
Several ingredients have heretofore been commercially employed as alumina stabilizers, namely, barium oxide, lanthanum oxide, and cerium oxide. These oxides are usually promoted by impregnating the catalyst substrate with a solution in which is dissolved a salt of such metals for such oxides. Calcination of such impregnated substrate results in a washcoat of the desired metal oxide.
Of such stabilizers, ceria also has the ability to absorb and store oxygen on the catalyst when an exhaust gas temporarily becomes oxygen-excess and to release oxygen when it becomes oxygen-short. Thus, if an insufficiency of oxygen, required for oxidation of CO or HC, should occur for a moment in a reaction gas, when ceria and a precious metal are intimately deposited on alumina, the reaction of the gas can be carried out by oxygen release from the cerium. This oxygen transfer characteristic of ceria further enhances the catalytic effectiveness of precious metals, particularly at and adjacent stoichiometric conditions.
To utilize these dual advantages of ceria, there has developed two distinct manners of use of ceria for oxidation catalysts: (i) use of finely divided CeO.sub.2 as a codeposit with a small quantity of precious metal resulting in a microscale intimacy between the elements in the resulting coating, represented by U.S. Pat. Nos. 3,850,847; 4,448,895; 3,993,572; and 4,367,162; and (ii) a first deposition of CeO.sub.2 onto the substrate followed by a separate deposition of the usually small quantity of precious metal, but still resulting in a microscale intimacy between CeO.sub.2 and the precious metal, such as represented in U.S. Pat. Nos. 4,448,895; 4,591,580; 4,407,735; 4,476,246; 4,426,319; 4,153,579; 3,903,020; 4,157,316; 4,331,565; 4,189,404; 4,283,308; and 4,294,726.
As part of the research leading to this invention, it has been discovered that the use of cerium oxide with precious metals in microscale intimacy results in two problems: (a) a lack of tolerance to lead poisoning, and (b) the suppression of catalytic activity of saturated hydrocarbons.
With respect to the increase in lead poisoning, this is not only a problem for engines supplied with lead-containing gasoline supplies, such as is prevalent in Europe and Australia, but also for engines adapted to run on leadfree gasoline since lead is typically present such latter gasolines in trace amounts. To convert lead oxide to lead sulphate in exhaust gases requires oxidation of SO.sub.2 to SO.sub.3 ; the presence of SO.sub.3 converts lead oxide to harmless lead sulphate. But the presence of SO.sub.3 is dependent upon the catalytic oxidation of SO.sub.2 by the precious metal. Unfortunately, the microscale intimacy of ceria with the precious metal suppresses such catalytic activity. With respect to the suppression of catalytic activity for saturated hydrocarbons, this phenomena appears to be the result of the inability of ionic PdO to oxidize saturated hydrocarbons and the inhibition of the reduction of PdO to Pd by the presence of CeO.sub.2.
Accordingly, a primary object of this invention is to provide a catalyst construction for automotive emissions, derived from fossil fuels, that will retain the advantages of ceria use and eliminate problems associated with ceria use.